Coen Welsh | Nov 14, 2017 | 0
The Week’s Weather up to Friday 26 May Five-day outlook to Wednesday 31 May
500 mB (18,000 feet) vorticity for Saturday 27 May 2017
Source: wxmaps.org, GrADS/COLA
During this week the synoptic map has moved to a conventional winter pattern. The South Atlantic high pressure cell and the southern Indian high are spaced roughly equidistant west and east of the sub-continent with the South Atlantic high somewhat stronger at a respectable 1032 mB. The southern Indian high only managed to registed 1024 mB. The cores of both straddle the 33°S latitude indicating a stable winter position.
In the absence of a strong low pressure system between the two highs, the cold front that passed the continent did not develop a northern extension. It passed a few hundred kilomtres south of the continent without any real impact on land. The next approaching frontal system will probably not be as severe as expected since the South Atlantic high is pushing in between the front and the continent, displacing the front to south and limiting its impact to the north.
However, in the upper atmosphere, conditions are remarkably different. The weather map for this week’s discussion shows conditions 18,000 feet aloft at the so-called 500 mB surface. From the map it can be seen that the front is much stronger higher up than on the surface. The blue band is where lateral air movement is at its strongest, indicating that the upper air over the southern half of Namibia, on Saturday and Sunday will be far more unstable and cold than the surface air. The pink areas indicate upper-air ridging, a hallmark of high pressure control in the upper levels.
Although 18,000 feet is only about half-way up in those layers of the atmosphere that have an immediate influence on surface conditions, it as an important indicator of conditions both below and above it. When a cold front on the surface is situated south-west of the continent, its upper air extension will stretch many hundreds of kilometres to the north and the north-east. Since these fronts are typically on the leading (approaching) edge of the high pressure cell, they may be shielded on the surface while creating much more volatility higher up.
For the person on ground level, the front may have a limited, sometimes even unnoticeable effect, but the lateral movement in the upper levels, will take the cold air to locations far removed from the origin of the front.
Ahead of a frontal system, the airflow is from north to south, which is the reason why Namibia had such a mild pleasant week. But this is only on the surface. In the upper levels the airflow this week was due south, with a strong mobility in the whole system. This means that while upper level airflow was predominantly south, the zonal airflow in the highest levels (35,000 to 56,000 feet) shifted the entire system at a rapid pace from west to east. When this happens, there is no opportunity for the system to become stationary so while a frontal system may bring severe cold, the sensory effect is limited as the system quickly departs to the east.
Namibia is now in the intra-seasonal phase where the frontal systems show a typical winter pattern but the sunshine hours per day is still enough to counter its effect on the ground. The sun heats the earth’s surface sufficiently to make warm days, despite chilly nights.
From temperature maps it can be seen clearly that while the Karas and Hardap regions were generally cooler, the northern half still approached 30°C during the late afternoon.
The South Atlantic high makes landfall on the west coast during Saturday evening. The 1024 mB isobar reaches into the Namibian interior as far north as Windhoek. This will bring in a chilly night but it will be short-lived.
During Sunday, the eastern half of the subcontinent is under high pressure control meaning cold conditions but in Namibia, the cold effects only the south-western quadrant.
Conditions over the interior are static from Monday to Wednesday with cooler days and cold nights in the east, especially along the Botswana border.
The pressure differential between the interior and the coastal plain is minimal meaninig very little or no Oosweer effect but from Tuesday onwards, a low pressure system starts moving down the northern Namib, which may lead to mild Oosweer from Hentiesbaai to the Kunene River mouth.